Abstract : A heat exchanger is described for cooling contaminated liquids causing precipitation of solid matter on cold walls. Waste water at high temperature containing contaminants in the form of sludge and in solution is pumped through the annular space between two concentric cylindrical tanks. Cooling water is circulated through the tanks on either side of the annular space while scrapping devices rotate about the annular space preventing buildup of precipitate on the annular space walls.